This study will evaluate the hypolipidemic effects, safety and influence on cholesterol homeostasis of simvastatin at the maximal currently approved dose (40 mg per day) at two higher doses (80 mg per day and 160 mg per day) in patients with primary hypercholesterolemia. After a four week dietary baseline, patients will be treated in a random sequence with simvastatin at the three doses to be evaluated with each dose of drug being given for a period of six weeks.